BEIRUT, Aug 26 (Reuters) - U.N. weapons experts are due on Monday to inspect a site where poison gas killed many hundreds of people in Damascus suburbs, amid calls from Western capitals for military action to punish the world's worst apparent chemical weapons attack in 25 years.

Syria agreed on Sunday to allow the inspectors to visit the site. The United States and its allies say evidence has been destroyed by government shelling of the area over the past five days, and the Syrian offer to allow inspectors came too late.

Washington has faced calls for action in response to Wednesday's attack, which came a year after President Barack Obama declared use of chemical weapons to be a "red line" which would require a firm response.

Obama has been reluctant to intervene in Syria's 2-1/2-year-old conflict and U.S. officials stressed that he has yet to make a decision on how to respond. A senior senator, Republican Bob Corker, said he believed Obama would ask Congress for authorisation to use force when lawmakers return from summer recess next month.

Secretary of State John Kerry said in a round of phone calls to his foreign counterparts that there was "very little doubt" the Syrian government had gassed its own citizens.

The State Department said Kerry emphasised this in calls to the foreign ministers of France, Britain and Canada as well as to U.N. Secretary-General Ban Ki-moon and Russian Foreign Minister Sergei Lavrov.

Russia, a major ally of Syrian President Bashar al-Assad, has suggested rebels may have been behind the chemical attack and said it would be a "tragic mistake" to jump to conclusions over who was responsible.

The White House said Obama and French President Francois Hollande "discussed possible responses by the international community".

British Prime Minister David Cameron and German Chancellor Angela Merkel agreed that "such an attack demanded a firm response from the international community," Cameron's office said.

Syria watchers said the government's decision to allow the inspections may have been an attempt to stave off intervention.

"My view is that the Syrian government's apparent agreement to the U.N. inspection has been triggered by the growing possibility of military action," said Malcolm Chalmers, Research Director at the Britain's Royal United Services Institute.

"I think that is why they are doing it."

In London, Foreign Secretary William Hague said evidence of a chemical attack could have already been destroyed by subsequent artillery shelling in the areas or degraded in the days following the strike.

"We have to be realistic now about what the U.N. team can achieve," he told reporters.

CEASEFIRE DURING INSPECTIONS

The United Nations said Damascus had agreed to a ceasefire while the U.N. experts are at the site for inspections.

Syria confirmed it had agreed to allow access to the inspectors, who arrived in Syria to investigate smaller chemical weapons allegations just three days before the huge incident, which occurred before dawn after a night of heavy bombardment.

Medicins sans Frontieres said at least 355 people were reported dead in three hospitals from symptoms of poisoning. Assad's opponents have given death tolls ranging from 500 to well over 1,000.

The experts' mandate is to find out whether chemical weapons were used, not to assign blame, but the evidence they collect, for example about the missile used, can provide a strong indication about the identity of the party responsible.

If the U.N. team obtains independent evidence, it could be easier to build an international diplomatic case for intervention. Former weapons investigators say every hour matters.

The team has been waiting in a Damascus luxury hotel a few miles from the site of what appears to have been the world's worst chemical weapons attack since Saddam Hussein's forces gassed thousands of Iraqi Kurds in 1988.

Syria's information minister said any U.S. military action would "create a ball of fire that will inflame the Middle East".

He said Damascus had evidence chemical arms were used by rebels fighting to topple Assad, not by his government. Western states say they believe the rebels lack access to poison gas or weapons that could deliver it.

Assad's closest ally Iran, repeating Obama's own previous rhetoric, said the United States should not cross a "red line" by attacking Syria.

Two and a half years since the start of a war that has already killed more than 100,000 people, the United States and its allies have yet to take direct action, despite long ago saying Assad must be removed from power. (Additional reporting by Lesley Wroughton in Washington; Editing by Mohammad Zargham and Christopher Wilson)